By producing inflows, dissipation can dramatically deepen galactic
potential wells, and these deeper wells seem to influence the dynamics
of collisionless material (eg.,
Katz & Gunn 1991,
Udry 1993,
Dubinski 1994,
BH96).
But these studies mostly examined effects of dissipation
on dark halos; only the last one focused on disk-galaxy mergers, and
that work compared but one pair of carefully-matched simulations.

The two remnants compared by
BH96
were produced by mergers of
equal-mass bulge/disk/halo galaxies. Both experiments started with
exactly the same initial conditions, using disk inclinations
of 0° and 71°; both were evolved with the same spatial
resolution (a.k.a. ``force softening''). In the dissipative version,
a tenth of the disk mass was treated as gas with a cooling cut-off at
Tc = 104 K, while in the collisionless version
everything obeyed the collisionless Boltzmann equation.

Figure 3 compares the ellipticity profiles of these two
remnants. Beyond their half-light radii (rhl 0.18
model units) both remnants are nearly oblate and rotate rapidly in
memory of the direct (i = 0°) disks used in the initial
conditions. But inside rhl the two remnants are quite
different; the collisionless version is a triaxial ellipsoid rapidly
tumbling about its minor axis, while the dissipative version is fairly
oblate and slowly rotating.

How does dissipation influence the shape of merger remnants? The
dissipative remnant has a deeper potential well as a result of its
central gas cloud, which contains ~ 4.5% of the luminous mass,
or ~ 0.9% of the total. But the finite resolution of the force
calculation spreads this central mass over a radius of ~ 0.04,
rhl; thus compared to a black hole or singular logarithmic
potential, this mass may be ineffective at scattering box orbits
(Valluri, these proceedings). Moreover, the oblate shape of the
remnant seems to be established at the moment of the merger itself
instead of developing progressively from the inside out (Ryden, these
proceedings).

Thinking that the shapes of these remnants might be constrained by the
scarcity of box orbits, I constructed a composite mass model with the
density profile of the dissipational remnant and the ellipticity
profile of its collisionless counterpart, and used its potential to
evaluate the phase-space volumes of the major orbit families
(Barnes 1998).
While this composite offered fewer boxes and more z-tubes
than the collisionless remnant, bona-fide box orbits were present at
all binding energies. Thus self-consistent equilibria as centrally
concentrated as the dissipational remnant and as flattened as the
collisionless remnant may exist. However, some finesse is probably
required to realize such equilibria. Merging sows stars far and wide
across phase space; not all physically consistent systems may be
constructed with such a blunt instrument.

All of this work is based on only one pair of simulations, and the two
remnants compared by
BH96
may not be entirely typical. For example,
the pre-merger disks in these experiments developed bars, and the bars
in the dissipational version had significantly higher pattern speeds.
Thus when the disks merged, their bars had different orientations, and
this might influence remnant structure. Comparison of a larger sample
of collisionless and dissipative merger remnants is clearly warranted,
but sufficient computer power is hard to find. Meanwhile,
collisionless mergers between models of various central concentrations
may help expose the connection between density profile and remnant
shape (Fulton & Barnes, in preparation).